With the rapid proliferation of fiber optic communications systems, devices for processing optical signals have become increasingly important. Planar devices comprising optical waveguides fabricated on planar substrates offer a promising environment for receiving and processing signals from optical fibers. Such devices ultimately offer the potential of integrated optical and electronic signal processing on a single semiconductor substrate.
A typical proposal for planar optical devices involves disposing high silica optical wave guiding structures on Silicon substrates. But while high silica glass is suitable for use in optical fiber, it is not optimal for use on silicon. High silica glass is refractory, has a poor thermal expansion match with silicon, requires a long deposition time, and is a poor host for the rare earth dopants contemplated for a number of device applications.
The refractory nature of high silica glass means that high temperatures are generally required to produce an optical waveguide. For example, in the flame hydrolysis method for depositing a high silica core, the deposited particulates are typically sintered at 1200.degree. C. or more. Such temperatures would deteriorate electronic devices formed in the underlying silicon.
Other techniques for depositing high silica glass layers tend to be unduly time consuming. For example, chemical vapor deposition grows only fractions of a micrometer of high silica glass per hour, and high pressure oxidation of silica on silicon typically requires up to 30 hours to generate 15 micrometers.
Other glasses, such as sodium-boro-silicate glass, can be formed in thin films, but it is difficult to make such films that are suitable for optical waveguide applications. Waveguides typically require a smooth outer surface, an essentially bubble free layer having a low density of bubbles (typically less than 10 bubbles/mm.sup.2), and--if bubbles cannot be entirely eliminated--bubbles as small as possible. The applicants have observed that prior processes for rapidly making thin, smooth films of glass, such as those disclosed in U.S. Pat. No. 3,212,921 to W. A. Pliskin, produce smooth surfaces at the expense of a high bubble density. Pliskin et al. heat glass powders for a short time (less than one hour), to produce a smooth surface. This is because at the low temperatures and short times used in Pliskin, bubbles rise near the surface but do not penetrate the surface. Thus the Pliskin films exhibit unacceptably high densities of entrapped bubbles (typically on the order of 200 bubbles/mm.sup.2). In addition, bubbles as large as 10 microns in diameter are observed. Applicants have further discovered that longer heating times on the order of two to eight hours do not Solve the problem. While longer times reduce the concentration of bubbles, they produce a rugose outer surface unsuitable for waveguides. Accordingly, there is a need for an improved method for making planar waveguide devices.